library.c

#include "library.h"

void LibraryFunction()
{
  int success = 1;
  if (success)
  {
    LogPrint("Success!");
  }
  else
  {
    LogPrint("We messed up.");
  }
}

library.h

#ifndef LIBRARY
void LogPrint(char *String);
void LibraryFunction();
#define LIBRARY
#endif

user.c

#include "library.h"
#include "library.c"

void LogPrint(char *String)
{
  puts(String);
}

int main(int argc, char const *argv[])
{
  LibraryFunction();
  return 0;
}